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The Journal of Neurophysiology Vol. 88 No. 2 August 2002, pp. 888-913
Copyright ©2002 by the American Physiological Society
Department of Psychology, University of Texas, Austin, Texas 78712
Albrecht, Duane G.,
Wilson S. Geisler,
Robert A. Frazor, and
Alison M. Crane.
Visual Cortex Neurons of Monkeys and Cats: Temporal
Dynamics of the Contrast Response Function. J. Neurophysiol. 88: 888-913, 2002. Cortical neurons display two
fundamental nonlinear response characteristics: contrast-set gain
control (also termed contrast normalization) and response expansion
(also termed half-squaring). These nonlinearities could play an
important role in forming and maintaining stimulus selectivity during
natural viewing, but only if they operate well within the time frame of
a single fixation. To analyze the temporal dynamics of these
nonlinearities, we measured the responses of individual neurons,
recorded from the primary visual cortex of monkeys and cats, as a
function of the contrast of transient stationary gratings that were
presented for a brief interval (200 ms). We then examined 1)
the temporal response profile (i.e., the post stimulus time histogram)
as a function of contrast and 2) the contrast response
function throughout the course of the temporal response. We found that
the shape and complexity of the temporal response profile varies
considerably from cell to cell. However, within a given cell, the shape
remains relatively invariant as a function of contrast and appears to
be simply scaled and shifted. Stated quantitatively, approximately 95%
of the variation in the temporal responses as a function of contrast
could be accounted for by scaling and shifting the average poststimulus
time histogram. Equivalently, we found that the overall shape of the
contrast response function (measured every 2 ms) remains relatively
invariant from the onset through the entire temporal response. Further, the contrast-set gain control and the response expansion are fully expressed within the first 10 ms after the onset of the response. Stated quantitatively, the same, scaled Naka-Rushton equation (with the
same half-saturation contrast and expansive response exponent) provides
a good fit to the contrast response function from the first 10 ms
through the last 10 ms of the temporal response. Based upon these
measurements, it appears as though the two nonlinear properties,
contrast-set gain control and response expansion, are present in full
strength, virtually instantaneously, at the onset of the response. This
observation suggests that response expansion and contrast-set gain
control can influence the performance of visual cortex neurons very
early in a single fixation, based on the contrast within that fixation.
In the DISCUSSION, we consider the implications of the
results within the context of 1) slower types of contrast
gain control, 2) discrimination performance, 3)
drifting steady-state measurements, 4) functional models
that incorporate response expansion and contrast normalization, and 5) structural models of the biochemical and biophysical
neural mechanisms.
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